Not really reasonable. That would 1) Make routing inefficient because routers have parse an additional, non-adjacent, non-contiguous header to get the source and destination addresses. 2) Break compatibility because there would exist "routers" that do not understand ipv6 headers. They receive your ipv4 with v6 packet and send it somewhere else.

The result is basically the same situation we are in today, except much more hacky. You'd still have to do a bunch of upgrades.